The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient's body lumen, such as a blood vessel, to maintain the patency thereof. Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the results of the procedure and reduce the possibility of restenosis.
Stents are generally cylindrically-shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway there through.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted into a compressed state for deployment into a body lumen. One of the difficulties encountered in using prior art stents involve maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery and accommodate the often tortuous path of the body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from shape memory metals or super-elastic nickel-titanum (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the blood vessel. Such stents manufactured from expandable heat sensitive materials allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent.
Expandable stents are delivered to the target site by delivery systems which often use balloon catheters as the means for delivering and expanding the stent in the target area. One such stent delivery system is disclosed in U.S. Pat. No. 5,158,548 to Lau et al. Such a stent delivery system has an expandable stent in a contracted condition placed on an expandable member, such as an inflatable balloon, disposed on the distal portion of an elongated catheter body. A guide wire extends through an inner lumen within the elongated catheter body and out its distal end. A tubular protective sheath is secured by its distal end to the portion of the guide wire which extends out of the distal end of the catheter body and fits over the stent mounted on the expandable member on the distal end of the catheter body.
Some prior art stent delivery systems for implanting self-expanding stents include an inner lumen upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner lumen to “uncover” the compressed stent, allowing the stent to move to its expanded condition into the target area.
In many procedures which utilize stents to maintain the patency of the patient's body lumen, the size of the body lumen can be quite small which prevents the use of some commercial stents which have profiles which are entirely too large to reach the small vessel. In particular, often in PTCA procedures, the stenosis is located in the very distal regions of the coronary arteries which often have small diameters. Many of these distal lesions are located deep within the tortuous vasculature of the patient which requires the stent to not only have a small profile, but also high flexibility to be advanced into these regions. As a result, the stent must be sufficiently flexible along its longitudinal axis, yet be configured to expand radially to provide sufficient strength and stability to maintain the patency of the body lumen. Since many commercial stents lack both the low profile and extreme flexibility needed to reach such distal lesions, they are not available for utilization for such procedures.
What has been needed is a stent which has a low profile and a high degree of flexibility so that it can be advanced through tortuous passage ways of the anatomy and can be expanded within the body vessel to maintain the patency of the vessel. Additionally, the expanded stent must have adequate structural strength (hoop strength) to hold the body lumen open once expanded. Such a stent should also have sufficient radiopaque properties to permit it to be sufficiently visualized on external monitoring equipment, such as a fluoroscope, to allow the physician to place the stent in the exact target location. The present invention satisfies these and other needs.